It's not that rare, I got it most of the time after I kept bashing into the crystals after the point of no return... at least you can escape from it.
Did I mention it only appears after the point of no return? That's probably why people think it's really rare.

Why was this enemy even included in this game, its the only fiend that isn't boss that drops lvl 4 key spheres and they are pretty much useless beyond the point of no return, sure they might help you a little but you can't get back so why? Now i have to either steal them from Nemesis, costing 25 000 for 1 fight, which isnt big ammount or bride Chimera Brain for about 250 000 for 2, why coudn't they just put some monster with 1 000 000 hp and make it drop just 1 lvl 4 key sphere as a rare drop instead of having to bride 2 for more than 125 000 each or try to steal them from nemesis which takes ALOT of time to even get to